Royal Society studies options for UK plutonium stockpiles

24 September 2007

The Royal Society, the UK's national academy of science, has published a report on the options available to deal with the country's stockpile of reactor-grade separated plutonium.

The report says that the UK's civil stockpile of separated plutonium now totals over 100 tonnes and has almost doubled in the past ten years. The stockpile is largely the by-product of commercial reprocessing of used nuclear fuel from UK power plants.

According to the Royal Society, the potential consequences of a major security breach or accident involving the separated plutonium are so severe that the government should urgently develop and implement a strategy for its long term use or disposal. The report recommends that such a strategy should be considered as an integral part of the energy and radioactive waste policies that are currently being developed.

The report suggests that the best current option is to convert the stockpiled plutonium into mixed-oxide (MOX) fuel, which can then be used as fuel in nuclear power reactors. It suggests that the plutonium would then be harder to steal due to the higher radioactivity of used fuel, which would require reprocessing in order to obtain weapons-usable plutonium.

The Royal Society says that if the government decides to construct a new generation of reactors, then the entire stockpile could be burnt as MOX fuel in those units. However, if no new reactors are built, a small proportion of the plutonium stockpile could be transformed into used fuel by adapting the Sizewell B pressurized water reactor (PWR) to burn MOX fuel. The report recommends that the remaining separated plutonium should then be converted and stored as MOX fuel pellets.

In the long term, the report suggests, the best option of disposing of the separated plutonium stockpile would be to bury it deep underground in the form of used fuel or, less ideally, MOX pellets. It is essential that the government's strategy for developing such a repository for nuclear waste includes an option for the disposal of separated plutonium and materials derived from it. However the report stresses the urgency of the government developing a strategy for dealing with separated plutonium in the meantime since, according to the Nuclear Decommissioning Authority (NDA), disposal sites for high-level waste may not ready until around 2075.

The UK is committed to maintaining reprocessing and MOX fuel production at Sellafield to fulfil existing contracts to reprocess used fuel from overseas light-water reactors (LWRs), the report says. The THORP reprocessing plant will require approximately three years of operating time to complete these contracts, but it is not currently operating due to waste discharge problems. The restart is scheduled for late-2007 and closure for 2010. Reprocessing in the Magnox plant is scheduled to end in 2012, after which the UK will have no capability to separate additional plutonium, thereby placing a cap on the size of the stockpile. The government has concluded that waste management plans and financing for any new nuclear power stations that might be built in the UK should be based on a once-through cycle in which used fuel will not be reprocessed. The stockpile of separated plutonium should therefore slowly decrease after 2012 as its overseas-owned material is converted into MOX fuel and repatriated to its owners.

The Sellafield MOX Plant (SMP) was built to convert the plutonium separated by THORP into MOX fuel. All plutonium from overseas reactors is scheduled to be returned in this form. The first MOX fuel was returned to overseas clients in Switzerland in 2005. SMP was designed to produce some 120 tonnes of MOX annually, but is not expected to achieve a production rate of over 40 tonnes. As a result, conversion of plutonium into MOX for foreign customers may continue until around 2022-2023.

Professor Geoffrey Boulton, chair of the report's working group, said: "The status quo of continuing to stockpile separated plutonium without any long term strategy for its use or disposal is not an acceptable option. The Royal Society initially raised concerns about the security risks nine years ago and we have not seen any progress towards a management strategy. Furthermore, the stockpile has grown whilst international nuclear proliferation and terrorist threats have increased."